[Luff]

Download and unzip NW.js, create a file called package.json with the following text:

  { "name": "JSPaint", "main": "https://jspaint.app/" }

Place it in the extracted folder, then start nw.

[barbs]

I've not used NW.js before but I'm guessing that won't work offline?

[Luff]

It works if the page is a PWA with offline support. But no, JSPaint doesn't seem to be one :/

You could probably use a local caching proxy if your love for MSPaint is true and pure.